I quote from the Zillow listing:
Please read carefully before scheduling showings. May not qualify for financing. Great "diamond in the rough" investment property or primary home needing separate apartments. Little is known about condition except that property has active roof leaks. Property is being sold "as-is" with no repairs, no clean-up, and no warranties expressed or implied. Upstairs apartment cannot be shown under any circumstances. Buyer assumes responsibility for the month-to-month tenancy in the upstairs apartment. Occupant has never paid, and no security deposit is being held, but there is a lease in place. (Yes, it does not make sense, please don't bother asking.)...More at the link, if you can get past the concept of buying a house with a permanent upstairs tenant.